Analog meters have been used in conjunction with sensors to provide visual display of the physical parameter that the sensor is designed to measure. Even though digital meters are now available, analog meters are still used widely, especially for situations where use of electricity is to be avoided for the reason of preventing electrical spark near the fuel storage tanks. These meters are usually installed in widely separated geographical regions so gathering the meter information requires wide-area communication network. Fortunately, the cost of sending compact digital data such as the meter reading or meter conditions is becoming very inexpensive nowadays. One example is the use of Short Message Service of the wireless GSM network. Therefore it is desirable to equip an analog meter installed in the field with a “non-contact” device that is capable of reading the meter digitally and sending the data or other conditions to a user or a computer system on the wired or wireless communication network. The reading device must be flexible to read different meters and must be able to perform a precise reading at reasonable time.
Previous image processing work by Robert Sablatnig et al.; “Automatic Reading of Analog Display Instruments”, Proc. of the 12th International Conference on Pattern Recognition (1994), pp 794-797 as well as “Machine Vision for Automatic Calibration of Analog Display Instruments”, SPIE Proc. on Machine Vision applications in Industrial Inspection III, vol. 2423 (1995), pp 356-366 described methods to read the analog utility (gas, water or electricity) meters. Utility meters differ from the gauge-like meter. First, a utility meter has multiple dials. Each dial has a needle that reads one digit (0-9) of the entire reading. In contrast, the gauge-like meter has only one dial or scale with fine reading resolution ranging from 1 out of 50 or 1 out of 1000. Second, the needle of a utility dial rotates continuously in one direction only. The needle of the gauge-like meter can only rotate in less than 360 degree and in both directions. Sablatnig et al. use the Hough transform technique to detect and locate the outlining circles of all dials in the utility meter. There is no detection of the graduation marks of the dial. The basic operation of the Hough Transform is described in Ballard and Brown: “Generalizing the Hough transform to detect arbitrary shapes”, Pattern Recognition Vol. 13(2) (1981), pp 111-122 and E. R. Davies: “Machine Vision: Theory, Algorithms, Practicalities”, Academic Press (1996).
U.S. Pat. No. 5,013,154 describes a “System for remotely reading an analog meter”. The teaching of this patent uses a video camera and an information processing system with look-up table to read gauge-like analog meters. The patent assumes that a pre-defined path of interest on the image plane is known. The information processing system extracts the intensity profile along the pre-defined path from the image and detects the location of the needle pointer based on the reflectivity difference between the needle and the background. A relative distance along the defined path between the needle and the starting point of the defined path indicates the reading. One assumption that the patent makes is that there is no black character, graphics or scale mark along the path of interest that can interfere with the black needle. The patent also assumes that the position and orientation of the meter is fixed with respect to the imaging system.
U.S. Pat. No. 5,673,331 teaches methods for reading gauge-like analog meters from video images. It also assumes that the position and orientation of the meter is fixed. Using a pre-operation calibration procedure to determine data such as the position of the needle pivot point and the leftmost and rightmost points of travel of the needle, the system performs the reading by locating the angular position of the needle pointer and compares it with the calibrated data. It uses a 2D template matching technique to determine the angular position of the needle.
U.S. Pat. No. 5,559,894 describes methods for inspection and reading of utility meters (not gauge-like meters). It first uses pre-defined templates to identify a particular meters and determine its relative position and orientation with respect to the fixture. From the position and orientation data, the angular positions of all the dial needles are determined. Again, one dial decides one digit of the meter reading.
It is an objective of the invention to provide a method for monitoring an analog meter which is flexible and allows easy installation for various existing analog meters.
It is another objective of the invention to provide an apparatus to perform that method so that flexible installation and remote monitoring of various analog meters is allowed.